In order to present images on a display unit of this type, it is conventional practice to read image data from a suitable storage medium, for example a CD-ROM, a hard disk, a server or the like and to store them in a buffer memory. A graphics accelerator accesses the image data stored in the buffer memory and moves them to a frame buffer. A display refresh unit (“Screen Refresh Unit”) accesses the image data stored in the frame buffer in order correspondingly continually to refresh the content of the display unit.
Graphics cards of the kind used for computer applications, for example, realize the scrolling of the image content presented on the display unit by using a high-performance graphics accelerator which accesses the frame buffer with a high bandwidth. The scrolling is carried out pixel by pixel. By way of example, if the image presented on the display unit is scrolled in the horizontal direction, the entire image content presented on the display unit is scrolled pixel by pixel in the horizontal direction. The consequence of this is that, at the left-hand edge of the display unit, a pixel column has to be filled with new image data which are read from a suitable storage medium, for example a CD-ROM. If the display unit is, for example, a screen with 1024×768 pixels, the content of the frame buffer has to be refreshed 1024 times for the scrolling of a complete screen content. Assuming that the scrolling is to be concluded within a time period of is and the graphics card operates in the 256 color mode (i.e., each pixel is represented by one byte), the graphics accelerator requires the following theoretical bandwidth:1[byte/pixel]×1024×768[pixels]×1024[1/s]=0.805 GB/s  (1) In computer applications, the corresponding monitor carries out temporal subsampling in the range typically of 50 Hz-120 Hz of the image output by the graphics card. This reduces the bandwidth required in practice compared with the theoretical bandwidth to a minimum of 39.1 MB/s (=1[byte/pixel]×1024×768[pixels]×50[1/s]) for subsampling at 50 Hz and a maximum of 94.3 MB/s (=1[byte/pixel]×1024×768[pixels]×120[1/s] for subsampling at 120 Hz.
The advantage of this solution is that the scrolling range is not restricted to the width of the frame buffer. On the other hand, the disadvantage is that a high-performance graphics accelerator with a very high bandwidth is required.
For navigation systems of the kind used in motor vehicles, a graphics accelerator with a high bandwidth has likewise been required hitherto in order to be able to scroll the representation of a map gently across a corresponding display unit. In order to obviate a graphics accelerator with such a high bandwith, a solution has been proposed for this area of application in which no memory transfers are necessary and, consequently, the overall system costs, the electromagnetic emission and the power consumption are reduced. In accordance with this solution, an image area, also referred to as a “scroll layer” or “map layer”, is defined, the dimensions of which in the horizontal and vertical direction are larger than the visible image area of the display unit. The position of the visible image area of the display unit within this larger image area is defined with the aid of a so-called pointer, the pointer designating for example the (xy) position of the top left corner of the visible image area of the display unit within the said larger image area. Consequently, the image that can be presented on the display unit can be scrolled in the horizontal and vertical direction simply by changing the pointer, without memory transfers being necessary for this purpose. Whereas the scrolling is carried out in particular by the graphics accelerator in the case of the above-described first solution for computer applications, the scrolling is carried out by the display refresh unit (“screen refresh unit”) in the case of this second-mentioned solution. However, the disadvantage of this second-mentioned solution is that the scrolling range is restricted by the width of the frame buffer.